S.T.A.L.K.E.R.: Shadow of Chernobyl
Patches
Patch is currently at version 1.0005. It is heavily recommended to patch to the latest version.
Float32
S.T.A.L.K.E.R.'s original shader code was and still is horrendous. Float32 adds many new features while increasing performance. It changes nothing about gameplay or looks (except high-quality effects, it doesn't change any models or textures) and is widely considered a must.
Complete Mod 2012
It is also recommended to download the Complete Mod. It contains major bug fixes, A.I improvements, upgraded shaders, new higher resolution textures, sounds, music, weather and visual effects. Without hindering the feel of the original game, so it can also be used by new players.
Zone Reclamation Project (1.7 XR2)
A bug fix mod that tries to keep the game as vanilla as possible, or not depending on how you set it up.
Where to buy
Tweaks
Removing Head Bob: Navigate to C:\User\Documents\stalker-shoc Open the User.LTX file. Change cam_inert to 0 (or more if you only want a slight head bob.)
For a great guide on tweaking S.T.A.L.K.E.R. go here.